"The Trouble With Tribbles"
A dispute over control of a planet brings the Enterprise to a space station, where they must deal with Klingons, edgy Federation officials, and a previously-unknown species of small, unbearably cute, voraciously hungry and rapidly-multiplying furry creatures. The tribbles were instrumental in foiling a Klingon plan to poison a shipment of quadrotriticale intended for Sherman's Planet, when those that had fed on the grain were found dead.
Nilz Baris: There must be thousands of them!
Captain Kirk [Buried Up To His Neck]: Hundreds of thousands.
Spock: 1,771,561. That's assuming one tribble, multiplying with an average litter of 10, producing a new generation every 12 hours over a period of three days.
Spock [Of The Tribbles]: They remind me of the lilies of the field. They toil not, neither do they spin. But they seem to eat a great deal. I see no practical use for them.
Doctor McCoy: Does everything have to have a practical use for you? They're nice, they're soft and they're furry, and they make a pleasant sound.
Spock: So would an ermine violin, Doctor, but I see no advantage in having one.
Tribbles (Polygeminus grex) were small, non-intelligent lifeforms originating from Iota Geminorum IV. Known for their prodigious reproductive rate, these round, furry creatures emitted cooing sounds while touched, which had a tranquilizing effect on the Human nervous system. Born pregnant, a single tribble with sufficient food could quickly increase its number exponentially through presumably asexual reproduction, bearing an average litter of ten every twelve hours. On their homeworld, tribble populations were kept in check by a large number of reptilian predators.
Doctor McCoy: Do you know what you get if you feed a tribble too much?
Captain Kirk: A fat tribble.
Doctor McCoy: No. You get a whole bunch of hungry little tribbles.
Captain Kirk: Well, Bones, all I can suggest... is you open up a maternity ward.